Modelling the effects of passing ships

MODELLING THE EFFECTS OF

PASSING SHIPS

Spencer J. M. A., McBride Dr. M. W:Ikresford P. J.,

Goldberg D.G.

HR Wallingford

Deift Univorsity of Technc1oy

Sdp HyromZflCS

Lzbr3tOr1

LIrry

Mekelweg 2 - 2628 CD DeIft

The Netherlands

phone: 31 15786373- Fax:31 15 781836

Reproduced from a paper presented at the

International Colloquim on Computer Application in Coastal and Offshore Engineering, Kuala Lumpa, June 1993

Modelling the effects of passing ships

Spencer J. M. k, McBride Dr .M. W., Beresford P. 1., Goldberg D. G.

Reproduced from a çxper presented at the

International Colloquim on Computer Applications in Coastal and Offshore Engineering,

Kuala Lumpa, June 1993

HR Wallinglord

Ad&ess and Retstorod OtTico: HR Wallingloid Ltd. HowboryPait, WaIkçtocd, Oxoì 0X1088A Tot: + 44 (0)1491 &35381 Fax: + 44 (0)1491 P2233

MODELLING THE EFFECTS OF PASSING SHIPS

J M A Spencer, HR Wallingford, Howbery Park, Wallingford, OXON 0X10 8BA, _UK Dr M W McBride, HR Wallingford, Howbery Park, Wallingford, OXON 0X10 8BA,UK

P J Beresford, HR Wallingford, Howbery Park, Wallingford, OXON 0X10 8BA, _UK D G Goldberg, Hydraulics and Water Research (Asia), 2721-A Jalan Permata 4,

_laman

Permata, 53300 KL

Abstract

When designing any coastal facility, two of the key questions that the engineer must address_{are as} follows:

- _{What effects will the facility have on the environment?} - _{What effects will the environment have on the facility?}

in recent years it is the former question that has dominated the thinking of

_{constructional} engineering. In our rapidly developing and consuming work, any new facility must be shown not to have a detrimental effect on the surrounding region or else the coastal environment_{would very} quickly deteriorate.

Nevertheless, in order for any _{new facility to be successful and to operate efficiently, the latter} question must also be addressed. Design _{standards do exist for the consideration of}

most effects

such as wind, waves, tidal levels, currents and so on. These standards tend for various _{reasons to} be idealised and conservative.

This paper describes new computer modelling techniques developed by HR _{Wallingford for} assessing the effects of a less frequently considered environmental effect on coastal works, those of passing ships. When a ship navigates along a channel, such as the approach to a port, large forces can be generated by the low pressure region abeam of the passing vessel. These forces_tend to draw any adjacent moored vessels toward that _{passing vessel. The magnitude of this pressure} disturbance is strongly dependent _{on the speed of the vessel and the layout of the port and its} approach. The effect on other ships depends on this distribution of pressures and the mooring configuration of the berthed ships. Typically, a berthed ship will be pushed close to itsquay at the

approach of the passing vessel, be attracted to the vessel as it reaches abeam and then pushed _back against the quay by the high pressure region in the wake of the passing ship. Substantial_differences in the force field over relatively small distances, particularly as the passing vesselmoves away from

the moored vessel, mean that the moored vessel will _{also be prone to large turning forces. The} combination of forces and moments on the moored vessel, particularly when coupled with ambient environmental forces such as wave, wind and tidal effects, can made loading and unloading operations difficult, inefficient and, in the extreme case, dangerous. Similarly, with the increased forces on mooring lines and fenders, berthing tenability might be compromised.

In a well designed port in an ideal location, passing ships would not have a significant effect_on those already in port. However, it is often the_{case that there is insufficient space or resources to} enable the perfect location for an approach channel _{to be found. Furthennore, port design might} be hampered by historical constraints or other legal or environmental considerations such as restrictions on dredging. Therefore_{an approach channel might by necessity be located in a position} where passing ships will have an effect on those berthed nearby. In such cases, it is_{important for} the safe and efficient operation of the port that the effect of the passing ships, _{combined with the} other environmental effects, be assessed so that suitable working conditions can be determined. When designing extensions to port facilities, particularly in more enclosed locations, it is important

for the basic feasibility of the new berths that these effects are considered. However, the use of standard design parameters can very often lead to wasteful and expensiveoverdesign because these

are based on idealised situations and necessarily include high safety factors.

This paper describes a suite of computational models that allows the effects of passing ships on moored vessels to be calculated, along with the effects of winds, waves and tidal currents. The models have been developed at HR Wallingford over a number

of years and are based on a

combination of in-house research work, published academic work,

field observation and measurement and physical model test results. The models produce as output the movement of the moored ships and the mooring forces. These allow downtime calculations and berthing tenability assessments to be made for the situation under examination. The suite of computational models is flexible and can easily allow a range of options to be tested, examining, for example, different maximum speeds for ships of given tonnage, different channel layouts and different berthing arrangements. Therefore, the suite can be used to help the port planner to produce an optimal port

layout, one that is operationally efficient, safe and as

cost-effective as possible, within any constraints that may be imposed.

The paper is illustrated by a recent application of the suite in an actual design project. The example demonstrates the general growing use of sophisticated mathematical models to simulate a variety of widely occurring problems, in this case one that is not frequently modelled. Such computer models allow studies to be made quickly and economically, indicating areas where operational difficulties may occur. It is through this type of modelling that potential problem areas can be identified and solutions reached early in the design of a development project at a stage when adjustments can still be made easily and with a minimum of disruption. Conversely, advantage can still be taken of favourable trends which are highlighted by the modelling, in order to improve operational efficiency of the ultimate design.

i

INTRODUCTION

The basis of the computational modelling techniques are essentially an application of Newton's Laws

of Motion. If it is possible to determine the inertia and the forces acting on a moored ship, then it is also possible to determine how it moves. The important forces acting on a moored ship include

mooring forces and hydrostatic buoyancy forces butthe most important ones are those that drive

the vessel 's motion, the hydrodynamic forces associated with the flow ofwater and waves around

the ship's hull.

in this paper, two types of hydrodynamic forces are considered associated with either wave action or with the flows caused by another large ship travelling past. Generally wave action might originate either from a distant storm or be generated in reaction to the motion of the moored ship. The hydrodynmic reaction forcesfrom the waves which the ship generates are commonly separated into components in phase with vessel velocity and acceleration. These are referred to as damping and added inertial forces respectively.

Five separate but inter-connected computational models are described in the following section. These are used to calculate the various forces on the moored vessel and then the consequent

movements. A flow chart showing the method of operation is given in Figure 1.

Initially the QUAYSHIP modei is used to calculated the hydrodynamic properties of the moored ship in the frequency domain. The output from this model is then used as inputfor the generation

of an Impulse Response Function (IRF) and wave forcing generation. This provides information suitable for describing wave forces on the moored ship for use in the SHIPMOOR model. _In parallel, the Passing Ship Model can be used to estimate the time histories of forcing on the moored

ship caused by a passing ship. Finally all these force effects are brought together, in conjunction with the effects of mooring lines and fenders and the moored ship's own inertia in the SHIPMOOR model. This computes how the moored ship will _{move with respect to time.}

2 _{DESCRIPTION OF COMPUTATIONAL MODELS}

2.1 _{QUAYSHIP model}

The QUAYSHIP model (Ref 1) is used to compute wave forces on a ship. It is a frequency domain model and is used to calculate forces on ships with regular periodic _{waves and movements.} Superposition principies are used to genet-alise to more complex cases. The model is based on

potential theory to describe the flow of water around the ship's hull and a linearised treatment of

waves. _{Its main limitation is that it does not include damping on the ship due to water viscosity,}

drag or vortex-shedding and loses _{accuracy when dealing with significantly non-linear waves.} Viscous and vortex-shedding forces are generally only significant in damping ship rolling motions and hence the model has a known tendency to give conservative (je over-large) estimates_{for roll} response.

The model is designed specifically for use in cases where the underkeel clearance, between the ship's hull and the sea-bed, is small compared to the ship's beam, the typical case for harbour studies. The QUAYSHIP model_{computes forces sufficient to desciibe all regular periodic motions} of the moored ship. Forcing due _{to incident wind-generated waves is calculated and includes the} effects of diffraction around the vessel _{and reflections off a quay wall. This isalso the case with} the damping and added forces due to the waves that the ship's own movement generates. The forces, however, are given _{a frequency domain representation whereas the SHIPMOOR}

model

ultimately requires a representation of forcing, damping and inertia as a time history. _{This is} achieved through the generation of the 1RF and wave forcing time history.

2.2 _{Impulse Response Function generation}

The added mass and damping coefficients of a moored ship are dependent on the periods of its movements. Therefore, if these _{are applied to anything other than regular periodic motions, the} simplest representation of hydrodynmic added inertia and damping forces in the time domain is likely to be erroneous. _{The HR model adopts a more sophisticated representation, the IRF.} Discussion of the mathematical theory on which the IRF is based is beyond the scope of this paper but it is derived from standard Fourier Theoiy (Ref_{2). A further value, the high}

frequency limit

value of the vessel's added inertia, is found from the IRF and added inertias

_{at different}

frequencies. The IRF and the frequency dependent added _{mass and damping coefficients are}

equivalent. They are different representations of the same force effects in different domains The IRF can be calculated from damping coefficients and vice-versa and this is used to check for consistency in the modelling process.

in physical terms, the IRF represents the time history of forces on a ship due to the_{action of waves} fter the ship undergoes an impulse movement. _{In idealistic terms, the ship}

_{staus at rest, is}

instantaneously brought into motion, moves a unit distance and then stops. This instantaneous, impulsive movement generates waves and these cause forces on the ship for _{some time after. The}

IRF is the time history of these forces. The IRF gives the_{wave damping and added inertia force} on a ship for any motion by means of a convolution type integral over the past history of _motion in the following way: if K(t) is the IRF, v(t) is the (past) velocity time history of_{the vessel and A} is the high frequency limit added inertia then the required force, F(r), is givenby:

dv

F(t) = -Aq.

- -

K(r) v(t-r dr

This expression is applied in the SHIPMOOR model to calculate forces and hence moored vessel

movement at the study site.

2.3

Wave forcing

If required, the forces on the vessel due to conventional progressive surface water waves, such as wind-generated waves at the site, can be calculated and forcing time sequences generated.

QUAYSHIP provides the required force values in the frequency domain. Thereafter, interpolation is used to estimate forces at extra frequencies, whenever necessary, to give forcing as a continuous function of frequency. Force time histories are then produced for a given wave energy spectrum

S(f) by the following Fourier Transform:

F(r)

= J fr(f)R(f)e

df

where Ê(f) is the complex representation of force on the ship due to regular unit amplitude waves at frequency f (from QUAYSHIP) and R(f) is a complex random variable with a uniformly

distributed phase and expectation value defined by:

E _(IR(f)I2df) S(f)4f

This calculation, incorporating the randomised value R(f) to represent random waves, gives a properly random forcing time series with the correct statistical properties. This represents forcing from waves with the specified input spectrum S(f).

I.ong period, second order forces can be significant exciters of moored ship motion, although the forces themselves are typically comparatively small. Their effects can be large when they act close to resonant periods of a ship on its moorings. In many cases one second order effect, namely set-down, dominates alE others and it is therefore important that it can be included into the simulation. Set-down (Ref 3) is a wave type disturbance of the water surface that is associated with the usual visible, first-order kind of water wave. lt is essentially a beating phenomenon and is always present in some form, although not usually visible because of its small amplitude, and its effect is to cause a depression in the mean water surface underneath groups of large first order waves. The

depression is in part caused by a reduction in mean water pressure. Therefore there is a pressure gradient underlying every wave group due to set-down which will exert forces on a moored ship and hence cause movement. This pressure distribution is, in many respects, just like any other surface water wave although of longer period than most wind waves. It can therefore be treated as such and the spectrum of the pressure, associated with set-down, can readily be calculated from the first-order wave spectrum:

1'

S,(f)

=

2j

_{y2S(f)S(f+r) df}

f2k.k

_{(1tanh.tanhkh)+}

k k2

rg

[f.f+f-)

_f+r)cosh2kh

f

coshhthj

2 4R2t

-

_{g(k-k) tanh(k-k)h}

where k andk are wave numbers satisfying:

4if2

gktanhkh and _{4i-2(f+f')2 =}

_gktanhk?i

With this spectrum it is now possible to calculate a force time series in the same randomised way as for first order forcing. First and second order forces_{are then summed and used as input to the} SHIPMOOR model. _{This treatment is not strictly exact as set-down and first order waves are} treated as independent whereas in reality they have a close but complex relationship. _However, more precise representation of that relationship will not add significantly _{to the accuracy of the} results. _{The approach described here gives the correct size for set-down forcing and hence the} correct size of ship response.

2.4

Passing Ship Model

The modelling of forces on a moored ship due to a passing ship requires different techniques from those used in QUAYSHIP and wave forcing. The difference _{arises because of 'non-wave-like'} disturbances created in the water by the passing vessel. Waves are created by passing ships, in the form of bow and stern waves, which are clearly visible if the ship moves at a significant speed. However, the very short wavelength and amplitude of these waves does not lead to any significant disturbance of a moored ship and are therefore not included in_{the modelling.}

The disturbance created by a passing ship in the water which _{causes significant moored vessel} movements is a pressure distribution associated with the flow of water around the moving ship's hull. Briefly, the ship needs to push water out of its way as it moves, this gives high pressure at the bow. The accelerated water then flows down the hull sides towards the stern and produces a

low pressure region. The water is then brought to rest again by high_{pressure around the vessel} stem. The effects on a moored ship are, in chronological order:

- _{repulsion as the high pressure field near the bow tends to force the ships apart while the}

passing vessel is approaching;

- attraction as the passing vessel draws level;

- _{strong attraction as the passing ship starts to drawaway. This is the phase when moored} ship movements and mooring forces are usually largest;

- _{repulsion as the effect of the stern high pressure is felt.}

The Passing Ship Model represents these effects using depth-averaged potential theory. That is, it computes a two-dimensional potential flow, necessarily neglecting vertical velocities around and beneath the vessels, that satisfies mass conservation criteria while _{allowing for the obstructive} effects of the hulls and seabed bathymetry. The flow induces _{a -pressure distribution around the} moored ship and from this, the forces and moments on the vessel are found. Since it is a potential

flow model, such effects as vortex shedding, turbulent wakes and viscous forces do not appear. Comparison with published measured physical model test results has shown the method to be accurate (Ref 2).

Seabed bathymetry at the site is represented in the model on a square grid, allowing inclusion of the constrictive and blockage effects of shallow water and the shoreline. Depth-averaged potential values are then calculated for each grid square. The moored ship is placed on the grid as another station2ry obstruction to flow. The passing ship forms a further obstruction with sources and sinks at bow and stern representing the vessel's displacement of water as it moves forwards. Progression of the moving ship and changing forces on the moored ship are modelled by stepping the moving vessel forward one grid square at a time, re-computing flows and forces at each step.

Output from the Passing Ship Model forms a time history of forces and moments on the moored ship as the passing ship travels across the model area. This constitutes the forcing input to the

SHLPMOOR model.

2í

SHIPMOOR model

The final and crucial stage of the modelling of ship movement is the calculation of the motions and mooring loads on the moored ship. This is carried out using the SHIPMOOR model. Typical

output values are maximum excursion distances for the moored ship along with mooring loads and plots to show time-variation of position and force. Examples are shown in Figures 2 and 3.

SHIPMOOR (Ref 2) is a time domain model. lt inputs the forcing time series obtained from the Passing Ship Model, the wave forcing calculations and the computed IRF, together qth data describing the ship's mooring lines and fenders, buoyancy forces and any steady wind or current loads that may be applied. Also included are the masses and moments and products of inertia. With this information the model can calculate total resultant forces and moments acting at any time, for any position and orientation of the ship. The model then proceeds to solve the vessel's equation of motion stepping through time, integrating at each stage to obtain velocities from accelerations and positions from velocities and similarly for rotational parameters. The integration scheme used is a high order predictor-corrector method with automatic time-step halving to ensure accurate

calculation.

Since the ship's motion has six degrees of freedom (three to specify its position in space, three to specify its orientation), it is best described using 6-vectors, .(t) for position and orientation and (t) for translational and angular velocities. If the positional components of, are taken as specifying

the position of the vessel's centre of mass, and all moments are taken about that point, then the ship's equation of motion may he written down in the matrix form SHIPMOOR uses:

d

x = V

dt-y = (A + M) F (F(t) + Gx, y) - I K(r).v(t-r) dr) I

d-

t

=

-

---

-where and M are 6x6 matrices containing the high frequency limit added inertia (computed with

(here expressed in matrix form), _{E is a 6-vector giving external forcing (translational and rotational)} on the ship (a sum of passing ship, _{wave, wind and current forces) and Q is also}

a 6-component

force and moment vector representing mooring and buoyancy forces.

Note that no assumptions in this formulation have been made regarding linearity of_{mooring forces.} The only linearity assumption implicit in SHIPMOOR is that the ship rotates through small_angles only as the form of the equation of motion given above becomes invalid for large rotational_angles. However the model is in other respects quite general _{if given suitable force data. SHIPMOOR} allows the input of non-linear fender and mooring line load curves, taking account of slack lines and non-contacting fenders as these do not exert forces.

Friction on fenders is also included. The mooring force is taken as being a function of velocity as well as the ship's position and orientation. This reflects the fact that friction acts in opposition to the local velocity of the hull surface. Fender friction is, though, hard to evaluate because reliable values for coefficients of friction are seldom available, particularly once the potentially _lubricant effects of water on hull and fender face are taken into account. This can be demonstrated to be a potent damper of excessive vessel movement which cannot, realistically, be ignored. A typical value must therefore be assumed

Although the equation of motion is given

_{for the case of the full six degrees}

_{of freedom,} SHIPMOOR is flexible and need not be used in that way. For example, in certain studies, tests of the moored ship movement due to a passing ship without additional wave action may indicate that only surge, sway and yaw movements of the larger ship would be of_{any significance. In these} circumstances, the SIIIPMOOR model can be run with the other three components inactivated _by being heEd at constant values. This has a large computational advantage in speeding calculation. In general use SHIPMOOR used in conjunction with QUAYSHIP, the 1RF, wave forcing and the Passing Ship Model has been found to give results that could be considered adequate to the requirements for accuracy in preliminary studies. A _{number of conservative assumptions have been} built into the models and the modelling procedures. _{Experience has shown that in general}

the

model predicts vessel movements -bigger than those which occur in reality. Results are therefore interpreted with this in mind and, for example, marginally _{unacceptable test results predicted by the} model are more likely to be acceptable in practice.

3 CASE STUDY

This section contains an example of the application of these modelling techniques at the preliminary

stage of a project design. The project involved the _{development of Kuala Lmnpur's Port Klang.}

Two berths were to be built to extend an already existing quay along a navigation channel, as shown

in Figure 4. Due to the potential number and_{size of ships that would be navigating the channel} to berths further north, the port authority_{was concerned about the possible disruption that}

these

passing ships would have on the vessels moored at the new development. In respect of ship movements and mooring forces of vessels at berth, the model was used to identify the _following:

Most sensitive quay position Critical passing ship type and track Critical passing ship speed

Most important water level and associated current strength conditions Downtime as a result of the passing ship

6. _{Additional downtime due to storm wave action (smaller vessel only).} 3.1 Methodology

Test conditions were selected with consideration to the factors most likely to affect the magnitude of the pressure field induced by the passing ship (le ship sizes, tide direction, strength of flow and water level). Several combinations of these parameters were selected to ensure that the worst-case scenario would be tested in the model as described below.

3.1.1 Vessel selection

Container ship with a 74000 tonne displacement

This container ship was selected to represent one of the moored ships and one of the passing ships As a moored ship it represents one of the largest ships likely to berth at the port. This allows the study to determine the maximum movements, mooring line and fender forces as a worst-case for a ship of this size. As a passing ship the 74000t container ship represents one of the largest ships to be travelling laden, and therefore at maximum draught, along the south-going track (475m from the berths), past the new development. When _{a ship is travelling at its maximum draught it will} induce the greatest pressure field due to its displacement. This condition will provide the maximum contribution to the movement of a ship moored on the berths from _{a ship passing on this track.} Because of its size wave forcing was not considered for this ship.

Container ship with a 6400 tonne displacement

The 6400t container ship represents one of the smaller container ships likely to moor at the berths. Tests carried out on this ship allowed the effects of the worst-case passing ship to be combined with the effects of wave action in the area. The peak period (Tr) of the storm wave conditions was specified as 60s, with a significant wave height (H1) of 1.Om.

Tanker with a 107000 tonne displacement

This tanker represents the worst-case vessel to use the north-going track (300m from the berths) fully laden. Tankers travel in-bound to off-load cargo at berths to the north of the proposed development and therefore only travel along the south-going track in ballast. With a displacement of 107000t, compared to the 74000t of the larger container ship, the tanker produces the greatest

pressure distribution, which will contribute to moored vessel movement, passing on the track closest

to the new development.

3.1.2 Test program/ne Tests

l-10 These enabled the worst-case conditions with respect to the berth position, passing ship, water level and tidal flow to be determined for one ship.

11-12 Based on the first ten tests, with a passing ship speed of 6kn through the water, the worst conditions were used to test the sensitivity of the moored ship to the speed of the passing vessel.

13-15 These allowed the critical crossing point of two passing ships travelling in opposite directions to be determined.

16-17 These allowed _{the effect of waves on the smaller ship to be}

determined

18-19 These examined the movements of moored ships with a ship passing_{berths. only 200m from the}

32

Setting up 3.2.1 Bathymerry

Depth information, derived

_{from drawings of the site, were digitized to cover an}

area of

approximately 3km2. The values were written to a file in (x,y,z) form and then_{transformed to be} compatible with the input required for the passing_{ship model. Depths}

were extracted on a 9m by 9m grid placed over the entire area.

3.2.2 Wind and

waves

Wind and current forcing

The wind forcing on the moored ship

was specified as a 30s wind gust of 361m. The most severe wind direction is beam_{on to the moored ship, off the quay. This results in the maximum}

wind force on the moored ship and reduces the friction effect of the fenders and_{allowing it the potential} to move more freely. The

forcing caused by the wind was estimated from the method detailed in the OCIMF report (Ref 4). The forces determined from these calculations_{were applied at all times} during testing to give the worst wind conditions _{possible at any time at the}

site.

The OCIMF report also deals with the forcing of the tidal flow_{on a moored ship. For this study} the current flows either bow or stern on and consequently the drag_{coefficients, and hence forcing,} were found to be negligible.

Wave forcing

Local storm wave conditions were defined as significant _{wave height (He) of i .Om with}

a peak period (T,) of 6.Os. A_{theoretical JONS WAP spectrum was assumed and the}

corresponding forcing

on the moored ship was derived from the wave forcing _{model to produce a wave forcing time} series.

3.2.3 Passing vessels

The passing ship model_{was run for each ship and each selected water level to determine the forcing} that would be induced

on the moored vessel. This was determined from the dimensions of _the moored and passing ships in conjunction with the bathymetry input file, the required _{water level} and the distance of the passing ship track from the berth.

For the tests involving two crossing ships, the passing _{ship model was used to model two vessels} travelling in opposite directions. This required specification_{of the position at which}

the passing

ships' midships crossed opposite a specified point on the moored ship (je bow, stern_{or amidships).} 3.2.4 Moored vessels

(a) _{QUAYSI-HP model runs and}

Impulse Response Function generation

Data files were _{set up and verified to describe}

the characteristics of the moored container _ships. These were used as input to the first stage of the

modelling process, the QUAYSHIP model. As

described in Section 2.1, the QUAYSHIP model _{was used to calculate the added mass and damping} coefficients for the moored ships over a range of_{frequencies. The output from this model}

was then

used as input for generation of an IRF which

was checked for consistency from the added_{mass and} damping calculations.

(b) _{Mooring and fender arrangement}

The mooring and fender_{arrangements used in the testing}

are shown in Figure 5. These_{were drawn} up according to information supplied in respect of_{mooring lines, bollards and fenders, along with} the recommendations on mooring arrangements given in _{BS6349 Part 4, Section 3 (Ref 5).} From the mooring information the details of the_{mooring line lengths and hence the stiffnesses were} calculated for each water level, along with the_{coordinates of each line with respect to the vessel's} centre of gravity. This

formed the mooring infonnation required by the SHIPMOOR model.

3.2.5 Equilibrium runs

For a ship moored_{against a quay in still water and calm conditions, the}

stiffness and pre4ension of the mooring lines, along with the action of the fenders, allows the ship_{to move to a steady state} or equilibrium position. Similarly, with the action of_{a steady force, such as that caused by a beam} wind, the ship will find a new equilibrium position according_{to the stiffness of the mooring}

lines

and the reaction forces of the fenders.

It is important in the use of the SHIPMOOR model that_{before each passing ship test is carried out,} the moored ship should be initially placed at its equilibrium _{position. This ensures that}

any vessel

movement induced by the passing ship can be seen to begin and end when _{the ship is at this steady} state position. This also allows_{the tests to be run much faster than if an equilibrium}

position had

to be reached during the test, before the passing ship forcing could be introduced. For these reasons

a series of equilibrium runs were carried out on each ship. These were also carried out for each water level, _{due to the differing lengths and therefore stiffliesses of the}

mooring lines and the line of action of the fenders on the ship's hull.

3.3

_{Discussion of results}

3.3.1 _{Criteria for discsioa of results}

The criteria generally used to assess the significance of vessel movements, with _{respect to cargo} handling operations, are limits for each mode of movement of_{a container vessel below which}

it is

thought that 90% to 100% cargo handling efficiency is possible. Examples of such limits _{(Ref 6)} are as follows: Surge 0.5m Sway 0.8m Yaw 0.5 Heave 0.45m Roll 3.00

Pitch

_l.5

It is these limits which have been used as a basis for discussion of the moored vessel _movements resulting from a passing ship and waves (when included).

3.3.2

_{740(X) tonne container ship moored on north and south berths}

With the74000t_{container ship passing along the south-going track the pressure field created by its}

motion through the water induces forcing on the moored ship. The forcing created by_{a south-going} vessel causes the ship to move as follows:

- _{Initially, as the passing ship approaches the stern of the moored vessel, it imparts a small}

negative sway and yaw motion due to the high pressure are around the bow of the ship. This leads to a very small movement of the moored ship due to the large distance from _the berth to the south-going track.

- _{As the passing ship starts to Iravel abeam of the moored vessel, the low}

pressure area induces a combination of positive surge, sway and yaw forcing. This causes the moored ship to travel northwards (astern) in surge along the berth and to move off all but the bow-most fenders due to the combination of the sway and yaw movements

-- _{When the passing ship is abeam of the moored vessel the positive sway forcing peaks} due

to the more uniform low pressure around the midship

_{area of the moored ship.}

Consequently the surge and yaw forcing become zero.

- _{As the passing ship travels on past the moored vessel the moored ship reaches its}

maximum

positive surge, sway and yaw position. Then the sway forcing begins to decrease and the ship starts to move back onto the fenders. At the same time the surge andyaw forcings become negative as the low pressure region created by the passing ship_{moves towards the}

bow. The ship starts to

_{move southwards (forwards) and the stern moves in toward the} fenders, whilst the bow is 'pulled' out and in_{the direction of the passing ship.}

As the high pressure area around the stern of the passing ship affects the_{moored ship, a} small negative sway and yaw motion occurs, similar to that imparted on approach.

Finally as the influence of the passing ship becomes negligible, the mooring line tensions, fender reactions and the steady wind force cause the moored vessel to move back to its equilibrium position.

With the 107000t tanker travelling along the north-going track a similar sequence of movements occur, only, due to the passing ship approaching the moored vessel's_{bow (rather than the stern with} the south-going vessel), the movements are effectively reversed in direction. Hence, the initiai movement is southwards, with the bow 'pulled' off the fenders, leading to a maximum movement in surge to the north, with the stern being pulled off the fenders, finishing witha small southwards

surge movement, as shown in Figure 2, for the conditions of Test 11. _{The initial and final positive} sway and yaw forces created by the high pressure areas around the bow and stern of the passing ship remain small

-Tests i _{to 10 identified that the north berth was marginally more sensitive to moored vessel}

movement than the south berth. _{This is a consequence of the slightly shallower water at the} northern end of the berth and across the channel generally off this location. Further, the_worst-case conditions were identified as low water with the 107000t tanker passing on the north-going track (300m from the berth) at 61m with the tide flowing in a northerly direction at 3kn.

3.3.3 Passing ship speed sensitivity tests

lt can be seen from the results of Test 11 (Table 1), with the 107000t tanker passing on the north-going track at lOkn, that a large maximum surge of O.88m was experienced by the moored ship. According to relevant literature, it is generally accepted that surging greater than 05m is liable to interfere with cargo handling operations. Therefore, in order to identifya passing ship speed which would not seriously affect cargo handling operations, Test 12 was carried out with a reduced passing

ship speed of 8kn.

The results of Test 12 showed that the movements and forces were all reduced to acceptable limits.

Notably the maximum surge of O.39m_{was below the adopted limit described above. Measurements}

were made of the duration of this movement, which was approximately 250s, from its _beginning until it became negligible. This duration is_{a characteristic of the passing ship speed through the} water and its corresponding speed with respect to the moored ship, taking the tidal flow velocity and direction into account. lt was noted that due to the faster ship speed through the water of 8kn, the duration of the maximum surge movement is shorter than the 330s_{surge duration from the} comparable test with a passing ship speed of 6kn. All of the other _{movements and forces remained} relatively small.

3.3.4 Crossing ship tests

Tests 13-15 studied two passing ships travelling in opposite directions, crossing _{at three points} opposite the moored ship _{The worst-case conditions, identified from Test 8, were used but with} a passing ship speed of 8kn for both of the passing ships. _{The results show that while the} movements and forces are of the same order as those with a single passing ship, the _worst-case

occurred when the ships crossed opposite the stern of the moored ship (compare Tests 13 to 15 with

Test 12 in Table 1). There was a marginal increase in movements for these conditions compared with the single passing ship, leading to a maximum surge of 044m. Thiswas due to the interaction

of the pressure distributions, with the most significant influence coming from the north-going (nearer) ship. Hence, the maximum moored vessel movement occurs as the north-going vessel travels past the stern of the moored ship.

Where the ships passed opposite the bow all movements were reduced when compared with those of the single ship. Again this is due to the interaction of the pressure fields on the moored ship starting at the beginning of the pass of the 107000t tanker. Finally, wIth the ships crossing opposite amidships the surge was even lower at O.26m but the sway and yaw were increased slightly due to the large pressure gradient caused by the interaction of the pressure distributions of both vessels

about the centre of the moored ship.

The results of these tests clearly show that for larger moored vessels an increase in the nominal passing ship speed from 6kn to 8kn (through the water) is viable.

3.3.5 Passing ship test at 2(X.)m from the berth

Test 19 was carried out with the 74000t container ship moored on the north berth. The worst-case

conditions of Test 8 were used with the 10700(k tanker passing at 8kn on a much closer track 200m

from the berth. This represents a ship that has stiayed from the north-going track. The results (Table 2) show a very' large surge movement of almost I .7m with significant movements in sway

33. I (0.8rn and 0.5°) for efficient_{cargo handling operations they nevertheless indicate}

a significant increase in moored ship response. The results confirm the sensitivity of the moored ship movement to passing ship distance from the berth and suggest that attention should be given to ensuring _that vessels keep to the defined sea tanes.

3.3.5 6400 tonne container ship _{- worst-case conditions}

Test 16 determined the movements and mooring forces resulting from the 107000t tanker passing on the north-going track (300m from the berth) at 8kn with a north-going tide of 3kn, at low water (+0.80m CD) and without wave activity. _{The vertical movements of this smaller ship were} determined in addition to the horizontal _movements.

The results of Test 16 show that all of the mooring forces and shin movements, With the _exception of roll, were very small. _{The reason for this is the relatively shallow draught of the smaller} container vessel. _{This results in a large underkeel clearance, even at low water, and a much} reduced underwater hull area, compared with the 7400(X container ship. The roll tends to be large due to the relatively low metacentric height of the ship combined with the relatively high mooring line positions at low water. This leads to large roll movements from _{only small forces applied} below the water line. _{In this case the motion was induced by}

the pressure distribution from the passing ship creating a positive rolling moment in conjunction with the wind forces,_{the effects of} the mooring line pre-tensions and reactions of the fenders.

3.3.6 Worst-case conditions including _{storm wave activity}

With the addition of the storm wave condition, of significant wave height (H) of i .Om and peak period (Tr) of 6.Os from a direction head on to the moored ship, Test 17 shows_{an expected general} increase in ship movements. Notable_{are the increases in horizontal movements. This}

is due to a combination of long period, second order effects (subharmonic motion, radiation pressure and set-down) which are known to influence moored ship response. However, with the exception of roll, the movements, mooring and fender forces all remain well below acceptable limits. En the case of roll, the generally accepted limit for 90% to 100% cargo handling efficiency is 3. It can be seen from Table 2 that the maximum roll, for the _{conditions of Test 17, was marginally above this} critical limit with a duration of approximately 1't0s. Therefore, it can be deduced that although the storm wave condition at the site contributes some effect to the overall horizontal _{movements of the} moored vessel, the roll motion induced by a passing ship constitutes the most significant _and potentially disrupting effect.

3.3.7 Passing ship test at 200m from the berth, _{including storm wave activity}

Similarly to Test 19 in respect of the passing vessel effect on the 74000t container_{ship, Test 18} demonstrates the effect of the I 070(» tanker straying from the 300m north-going track to pass the smaller moored vessel at a distance of 200m. _{This test was carried out with the storm wave} condition represented. Again, a general increse in movements and mooring forces_{is evident, with,} most notably, a 70% increase in roll, to give_{a maximum movement of 5.4. As with}

Test 17, the duration of the movement _{was approximately i 40s (determined by the speed}

of the passing shipand

the tidal conditions). The increase in roll is _{caused by the increase in themaximum rolling moment} induced by the closer passing ship, in conjunction _{with the low metacentric height}

and high mooring line positions.

3.4

Conclusion of the case study

The study concluded that, for the conditions tested, it could be assumed that if vessels passing_the berths travel at [east 300m from the quay at a maximum speed of 8kn (through the water),

movements and mooring forces of a vessel mooredon either the north or south berths should remain

within generally acceptable limits for 90% to 100% cargo handling efficiency.

4 CONCLUSION

This paper has presented the theory and usage of a new suite of mathematical models which can be easily applied to a situation which is, at present, under-modelled - the effect of passing vessels on moored ships. _{The suit has been shown to be complex enough to represent all the important} physical processes, including non-linear ones, and yet flexible enough to allow for a wide variety of cases and mitigation options to be tested. The suite is therefore a powerful tool in the hands of the port developer.

This example shows how computer models can be used in the field of coastal engineering to solve complex problems to a high degree of accuracy and within reasonable timescales and budgets.

5 _{ACKNOWLEDGEMENTS}

The authors would like to fh,nk the Klang Port Authority and their _{consultants Sepakat Setia} Perunding Sdn Bhd for their generous permission to use the results of the recent Passing Ships Study at Klang. The authors would also like to thank their colleagues at HR Wallingford for all of their help and advice regarding this paper.

6 REFERENCES

I _{Lean (i.H., Bowers E.C. & Spencer JM.A. QUAYSHIP: A computer model of a ship} against a quay in the presence of waves. FIR Wallingford. Report SR 232. February_1990.

2 _{Spencer J.M.A. Mathematical simulations of a ship moored in waves and of the effects of}

a passing vessel on a moored ship. HR Wallingford. Report SR 145. January 1988 3 _{Bowers, EC Harbour resonance due to set-down beneath wave groups. J. Fluid Mechanics}

Vol.79, p71.

4 OCIMF. Prediction of wind and current loads on VLCCs. Witherby & Co. Ltd, London 1977.

5 BS6349, Part 4 - Design of fendering and mooring systems. HMSO. 1985.

6 Thoresen, C.A. Port design. Guidelines and recommendations. TAPIR. 1988. ISBN

J TEST 11 12

13 14 ₁₅

_J

Passing ship; direction; speed (kn) Tanker N 10.0 Tanker N 8.0 Both-Stern N/S 8.0 Both-Bow N/S 8.0 Both-Mid N/S 8.0 Tide direction; speed

(kn) N 3.0 N 3.0 N 3.0 N _3.0 N 3.0 Surge (m) _{0.88 0.39} 0.44 _{0.33 0.26} Sway (m) _{0.11 0.02} 0.08 0.01 _0.04 Yaw () 0.08 0.02 0.05 0.01 _0.03

Maximum mooring line forces (t) with initial pre-tension _{of 20.Ot}

i Stern line _21.4 20.2 _{19.8 20.6} 19.8 2 Stern line _21.4 20.2 19.8 _{20.6 19.8} 3 Stern line _22.0 20.3 _{19.8 20.8} 19.7 4 Stern line _{22.6 20.4} 19.7 21.1 _19.6 5 After breasting _44.8 40.2 43.2 _{40.5 41.9} 6 After spring _27.9 23.7 25.2 _{22.6 23.0} 7 _{Forward spring} 21.6 20.2 _{19.8 20.7} 19.7 8 Forward breasting _{41.3 40.7} 40.9 40.3 40.5 9 Bow lIne _28.1 24.4 _{25.3 23.1} 23.5 10 Bow line _24.1 22.6 _{22.9 21.8} 22.1 11 Bow line _25.2 22.8 _{23.4 22.0} 22.2 12 Bow line _25.0 22.7 _{23.3 21.9} 22.2

Maximum fender forces (t) _{for pairs of main and}

intermediate fenders I - Stern _12.4 0.7 _{7.9 0.8} 4.4 3 12.1 1.3 _{8.2 1.3} 5.1 5 _11.8 2.1 _{8.4 1.8} 5.7 7 _{11.5 3.2} 8.7 2.5 _6.4 9 _{11.2 4.5} 9.0 _3.8 7.1 11 _{10.9 6.0} 9.3 5.1 _7.7 13 _10.6 7.9 9.6 7.3 _8.5 1 15 _{11.7 10.3} 10.8 9.7 _10.4 17 _{21.1 17.1} 17.7 13.1 _16.3 18 - Bow _{42.5 22.6} 33.2 _{15.2 24.4} H

Table I

_{Results of SHIPMOOR tests for the 74000t container ship moored at the north} berth

Table 2 _{Results of SHIPMOOR tests for}

74000t/6400t_{container ships moored}

_{at the}

north berth

TEST (74000t ship) 19 _{TEST (6400t} ship) 16 17 + waves 18 + waves Passing ship; direction; speed (kn) Tkr @ 200m N 8.0 Passing ship; direction; speed (kn) Tanker N 8.0 Tkr N 8.0 Tkr @ 200m N 8.0 Tide direction; speed (kn)

N 3.0 _{Tide dir; speed}

(kn) N 3.0 N 3.0 N 3.0 Surge (m) _1.69 0.41 Surge (m) Sway (ni) 0.05 0.16 0.23 0.25 0.20 0.54 Sway _Cm) Yaw (0) 0.28 1 Yaw «) 0.05 0.30 0.32

Mooring forces with

pre-tension of 20t

Heave (m) 0.04 0.06 0.08

i Stern line _{22.7 Roll () 2.55}

3.15 _5.35

2 Stern line _22.8

Pitch () 0.03 0.10 0.10

3 Stern line _24.1

Mooring forces (line pre-tension 4t) (t)

4 Stern line _25.3

1 Stern line

J

4.3 5.1 5.4

5-After breasting 63.4 2 Stern line 4.3 5.2 5.4

6 After spring _{35.8 3 Stern line}

4.4 5.3 5.4

7 Forward spring 23.7 _{4 Aft breasting 8.2}

11.0 12.6

8 Fwd breasting _{46.4 5 Aft spring 1}

3.9 4.5 _4.5

9 Bow line 35.5 _{6 Aft spring 2 3.9}

4.5 4.5 10 Bow line _{27.8 7 Fd spring 1}

4.4 5.2 5.3

11 Bow line 29.8 _{8 Fd spring 2 4.4}

5.2 5.3

12 Bow line _{29.4 9 Fd breasting}

7.6 11.8 11.4 Fender i - Stern 51.0 10 Bow line 3.7 5.0 _4.6

3 _46.8 - 11 Bow line _{3.8 5.2} 5.0 5 _{42.7 12 Bow line} 3.8 5.2 5.1 7 38.5 _{Fender 1 - Stern} 11.6 34.1 34.1 9 _34.3 2 5.4 14.2 15.7 11 30.2 3 2.7 12.0 13.3 13 26.0 I _2.8 11.4 13.7 15 22.3 5 4.0 21.7 22.9 ¡ 17 29.3

k

6 - Bow 77.7 .1 5.3 38.0 _45.2 18 - Bow

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